J.P. Crawford had two hits and drove in the only run for the Mariners, who have lost seven of nine in the midst of a playoff race.
Seattle fell 2 1/2 games behind first-place Houston in the AL West, and is a game behind Texas for the final American League wild card.
The Dodgers had several of their regulars out of the lineup after clinching the division with an extra-inning win Saturday night. Even so, Los Angeles still jumped out to an early lead.
Heyward hit a solo homer in the first inning, his 15th, and added a run-scoring single in the fourth.
“The finish line to us is, we want to be the last team on the field winning that game whenever the last game is,” Heyward said. “We just push every day.”
Barnes’ second homer of the season was a two-run shot in the second, giving Los Angeles a 4-0 lead. Outman capped the scoring with his 21st homer leading off the eighth.
“Disappointing way to finish the homestand, for sure,” Seattle manager Scott Servais said. “Today you’re hoping to get out on them early and get some runs and momentum early in the game, and we were unable to do that. They jumped on us.”
The Dodgers also got a strong pitching performance from left-hander Ryan Yarbrough (8-6), who held the Mariners to one run in 4 2/3 innings with seven strikeouts. Yarbrough came on in the second following opener Shelby Miller, and Gavin Stone fanned five in 3 1/3 innings of one-hit ball for his first career save.
“It couldn’t have worked out any better,” Roberts said.
Seattle starter Logan Gilbert (13-6) struggled, giving up five runs on seven hits in five innings. It was his first loss since June 28.
“This was a really big game, and we needed a better performance,” Gilbert said. “It’s frustrating. Not much I can do now, just try to clean it up. We’ve got big games ahead of us. It’s tough to say, but just move on and focus on the next one.”
END OF A STREAK
Seattle’s Teoscar Hernández was 0 for 4, ending his career-high 30-game on-base streak, which was the longest active run in the majors.
Dodgers who didn’t play included Mookie Betts, Max Muncy and Freddie Freeman. It was the first time this season Freeman wasn’t in the starting lineup at first base. He also didn’t miss a game last year until the Dodgers clinched the division crown.
Dodgers: RHP Lance Lynn (11-11, 5.94 ERA) will start the opener of a three-game home series against Detroit. Lynn is 5-2 since being traded from the White Sox in late July. He allowed two runs in seven innings last time out against the Padres.
Mariners: Travel to Oakland for a three-game series beginning Monday. RHP Bryan Woo (3-4, 4.16 ERA) will start the opener. The rookie threw six scoreless innings in his last start against the Athletics on Aug. 28.